Die stacking involves mounting one or more chips on another chip in a single semiconductor package. This process can increase the amount of circuitry that can be housed within a package of a given size, and thus reduces the real estate taken up on a printed circuit board by a chip. Die stacking may also simplify the assembly of printed circuit boards since multiple dies may be attached to a printed circuit board in a single operation. Die stacking also has the potential to improve the electrical performance of devices in which it is used since the interconnections between elements on each of the stacked dies may be shorter than the interconnections that would be required to connect the die elements on a planar surface. This can result in faster signal propagation and may also reduce cross-talk.
Connections may be formed between circuit elements on a first die and circuit elements on a die stacked on the first die by using through silicon vias (TSV's). As the name suggest, a TSV is a conductive via formed through a layer of silicon that provides electrical connections between elements on both sides of the via. A conventional die stacking arrangement using TSV's is illustrated in FIG. 1. A first die 100 includes a substrate 102 on which conventional layers of circuitry are formed and a top metal layer 104. A through silicon via 106 is formed in the substrate 102 by conventional processes, which may involve thinning the substrate to expose an end of the TSV 106, and a redistribution layer 108 is added to the side of the substrate opposite the conventional layers of circuitry. A microbump 110 may be formed on the redistribution layer to provide an electrical connection to the TSV 106. A second die 112 that is to be stacked on the first die 100 includes a substrate 114, a top metal layer 116, and a microbump 118. Electrical connections are formed between the first die 100 and the second die 112 by inverting the second die 112 and connecting the microbump 118 on the second die 112 with the microbump 110 on the first die 100.
The above-described arrangement may provide a satisfactory connection between stacked dies. However, it is not uncommon for TSV's to be defective. Defects may arise during the manufacturing process or may occur during use of the device embodying the TSV's. Thus, it is sometimes desirable to provide one or more redundant TSV's in parallel with a primary TSV to help ensure that at least one conductive path will be present even if TSV's are or become inoperative. FIG. 2, for example, illustrates a primary TSV 120 flanked by a first redundant TSV 122 and a second redundant TSV 124 in a conventional manner. As long as at least one of these TSV's is functional, a connection will be maintained. However, providing one or two redundant TSV's for each TSV used increases the amount of space required for connections and may increase the complexity of the dies. It would therefore be desirable to provide a connection between stacked dies using TSV's that does not require two or three redundant TSV's for each required die interconnection.